This happened to us a few years back on Celebrity, we specifically booked a large OV in the front of the ship corner close to our friends, the room was heaps bigger and we were so excited, it was an 11 day trip and the extra room and floor to ceiling window looked amazing. When we got to the terminal they had upgraded us to an aft balcony. We were not happy, it was noisy and loud and 1/2 the size of the room we had booked. They didn't care, apparently the people in the next cabin needed our cabin for a family trip because it had a connecting door. So my point is, it's NOT just MSC. I hope you still enjoy your cruise